Interferometers often utilize metrology signals that alternate sign as a moving arm (i.e., the porch swing) changes direction of motion. The moving arm creates an optical path difference; however, mechanical tilt about a rotative axis and/or mechanical tip about another rotative axis may be created as the moving arm moves back and forth. Two signals may be generated whose phase difference is proportional to the tilt (and/or tip). One of the signals (e.g., R) is known as the reference signal. The other signal (e.g., X), because of its physical relationship to the reference signal, changes phase sign each time the moving arm changes direction.
In certain interferometer systems, a dynamic alignment mechanism is is provided to compensate for the tilt and tip, for example, by counteracting the angular motion of the moving arm. Unfortunately, conventional phase detectors operate with signals whose phase does not periodically alternate sign. This is undesirable in certain interferometer applications that include the alternating sign metrology signal.
Attempts have been made to design interferometer systems that utilize a dynamic alignment control unit to determine the direction of the moving arm. This direction may then be input to an analog phase comparator. The analog phase comparator reverses the sign of its output each time the moving arm changes direction. As such, the output of the analog phase comparator may be considered to be proportional to the tilt (or tip) of the moving arm, and as such, it may be used as feedback to a servomechanism or the like.
Unfortunately, these attempts have not produced a phase detector that can accommodate a desired range of periodically alternating phase signals. For example, certain interferometer systems include an analog phase comparator with a limited range of ±120°. Further, these design attempts have not provided for an explicit signal that represents cavity tilt (i.e., the total tilt between the two arms of the interferometer).
Accordingly, it would be desirable to provide a more effective phase detector for use with periodically alternating signals to overcome one or more of the above-recited deficiencies.